This invention relates in general to the fixing of developed latent electrostatic images on a substrate, and more particularly, to apparatus for obtaining the desired permanent bonding of toner material, employed for development purposes in a xerographic copying machine, to the substrate.
In an automatic xerographic process of a type familiar to those skilled in the art and exemplified by U.S. Pat. No. 3,062,108, issued in the name of Clyde R. Mayo, the utilization of a heated fixing mechanism for achieving the permanent bonding of the developed latent electrostatic image onto the copy medium has proven highly satisfactory. One such fixing mechanism is commonly referred to as a fuser roll assembly. The fuser roll assembly additionally functions to feed the copy medium, such as paper, through the transfer station of the typical xerographic process. In providing the foregoing function, the fuser roll assembly cooperates with a backup roll. An example of such fuser roll assembly is disclosed in U.S. Pat. No. 3,291,466 issued to Aser et al.
In a typical construction of a fuser roll assembly, a hollow, generally cylindrical roll is mounted for rotation about its longitudinal axis, and is provided, along this axis, with an electric heating element. Such a roll is usually constructed of copper or aluminum and is normally provided with a coating of a suitable thermoplastic material, for example, polytetrofluoroethylene (hereinafter referred to as PTFE). PTFE is a fluorocarbon resin currently sold under the trademark "Teflon" by the E. I. duPont de Nemours and Company, Inc.
In operation, fuser roll assemblies are subjected to relatively high temperatures and pressures and this, together with manufacturing difficulties in obtaining perfect adhesion of the PTFE to the surface of the assembly roll, is the cause of deterioration of the PTFE coating which makes the roll unsatisfactory for continued use, thus necessitating its replacement. Where the roll is formed of copper, the roll is returned from the field, the PTFE being thereafter machined off (PTFE being almost chemically inert cannot be stripped easily without machining back to the base material). This machining operation causes loss of diameter beyond diameter tolerances, so the roll is deliberately machined slightly below true diameter and then built up oversize again, as by spraying with aluminum. The roll is then machined again to true diameter, and coated with PTFE as an original roll. This is a costly and tedious process.
When the roll is formed of aluminum, which is less expensive than copper, it is found economical to discard the aluminum rolls entirely when they can no longer be satisfactorily employed in copying machines. New roll assemblies are then substituted for the discarded assemblies. The difference in initial material costs plus the elimination of shipping costs from the field to the fuser assembly renovation site approximates the cost of repairing fuser roll assemblies including copper rolls. It is apparent that neither of these repair methods for fuser roll assemblies is satisfactory.